Accumulated Other Comprehensive Income/(Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	$ (533)	$ (913)	$ (592)	$ (995)
Other comprehensive income/(loss) before reclassifications	(155)	144	(116)	212
Amounts reclassified from accumulated other comprehensive (income)/loss	(96)	16	(76)	30
Other comprehensive income/(loss)	(251)	160	(192)	242
Balance at end of period	(784)	(753)	(784)	(753)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(522)	(914)	(587)	(994)
Other comprehensive income/(loss) before reclassifications	(254)	158	(189)	238
Amounts reclassified from accumulated other comprehensive (income)/loss	0	0	0	0
Other comprehensive income/(loss)	(254)	158	(189)	238
Balance at end of period	(776)	(756)	(776)	(756)
Derivative financial instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(11)	1	(5)	(1)
Other comprehensive income/(loss) before reclassifications	99	(15)	73	(27)
Amounts reclassified from accumulated other comprehensive (income)/loss	(96)	16	(76)	30
Other comprehensive income/(loss)	3	1	(3)	3
Balance at end of period	(8)	2	(8)	2
Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	0	0	0	0
Other comprehensive income/(loss) before reclassifications	0	1	0	1
Amounts reclassified from accumulated other comprehensive (income)/loss	0	0	0	0
Other comprehensive income/(loss)	0	1	0	1
Balance at end of period	$ 0	$ 1	$ 0	$ 1